hemistry 131 Lecture 10: Aldehydes and Ketones: Structure, Nomenclature, Physical Properties, and Reactivity hapter 16 in McMurry, Ballantine, et. al. 7 th edition W #5: 16.20, 16.28, 16.30, 16.32, 16.40, 16.44, 16.46, 16.52, 16.60, 16.62, 16.64, 16.68 The Structure of Aldehydes and Ketones The carbonyl: Notice how the resonance structure for the carbonyl tips us off to its chemical behavior: Aldehydes vs. ketones 2-propanone 3 acetone 3 ethanal 3 acetaldehyde dimethyl ketone Question: Acetone (propanone) is the smallest possible ketone. Is ethanal the smallest possible aldehyde? 1
Nomenclature IUPA Ketones: find the longest chain containing the carbonyl, use the carbonyl to establish numbering of carbons, drop the e and add one Lower the numbering to the carbonyl Aldehydes: find the longest base chain, drop the e and add al Again, the carbonyl establishes the numbering of the carbon chain Question: Numbering the functional group isn t important for aldehydes. Why? Name the following compounds 2
ommon Ketones: substituent, substituent, ketone (just like ethers or amines) Name the following compounds using the common nomenclature system ommon naming of aldehydes: treated like derivatives of acids (though that doesn t do us a great deal of good currently since we haven t discussed common naming of carboxylic acids). Acid: Aldehyde: formic acid 3 acetic acid propionic acid benzoic acid 3
The common nomenclature system makes sense when you consider the tendency of aldehydes to oxidize to acids: 3 3 3 Aldehydes and Ketones as Substituents If a more oxidized carbon is present (carboxylic acid or derivative) thus establishing numbering priority, the presence of a carbonyl may be indicated by the prefixes keto or oxo For instance, the glycolysis product pyruvic acid is also referred to as -ketopropanoic acid, 2-ketopropanoic acid, or - oxopropanoic acid, 2-oxopropanoic acid given the fact the common name for propanoic acid is propionic acid, there are many ways to get this naming job done! It is fairly common to name small aldehydes and ketones as substituents on aromatic rings by using the prefixes formyl (methanoyl) or acetyl (ethanoyl) R formyl 3 R acetyl Give 2 names for the following compounds 3 4
More nomenclature practice: 3 3 Physical Properties of Aldehydes and Ketones Refer to tables 15.2 & 15.3 Aldehydes and ketones have permanent dipoles: Thus, they boil higher than corresponding hydrocarbons or ethers. Note that they do not boil as high as the corresponding alcohols hydrogen bonding is a particularly strong dipole interaction 2-propanol BP = 103 o propanone BP = 58 o butane BP = 0 o As a 1 st approximation, aldehydes and ketones have the same physical properties For comparison, the boiling point of propanal is 49 o 5
The polarity of the carbonyl makes aldehydes and ketones good hydrogen bond acceptors; as an approximation, we can consider them to be as soluble as ethers and alcohols of the same molecular weight: Acetone 2 solubility @ 20 o 2-butanone (MEK) 2 solubility = 29 g/100 ml @ 20 o Reactions of Aldehydes and Ketones xidation Aldehydes are very easily oxidized. Ketones resist oxidation o It is not uncommon for a bottle of aldehyde to be contaminate with the corresponding acid due to air oxidation ; an example is benzaldehyde being oxidized to benzoic acid 2 benzaldehyde benzoic acid Recall primary alcohols are oxidized completely to acids, while secondary alcohols oxidize only as far as a ketone o You must have a hydrogen attached to the being oxidized for oxidation to occur 1 (recall tertiary alcohols are unreactive towards oxidation) The principal oxidizing agents in the lab are KMn4 and K2r27 2 KMn4 Mn2 (Mn +7 Mn +4 ), purple to brown change K2r27 r +3 (r +6 r +3 ), orange to green change 1 Excepting extreme conditions acetone is highly flammable 2 r 2 7-2 is more effective in an acidic environment as 2r 2 7; KMn 4 is more effective in base. Mixing dichromate with sulfuric acid gives 2r 2 7 and is referred to as Jone s reagent. 6
Question: an acetone (2-propanone) be used as a solvent when oxidizing n- butanol to butanoic acid with 2r27? Answer: In the body, aldehyde dehydrogenase uses NAD + as an electron acceptor to oxidize acetaldehyde (ethanal) + 3 Enz-Nuc: Aldehyde dehydrogenase Enz-Nuc 3 N 2 N + R + + NAD + Enz-Nuc 3 + NAD Enz-Nuc: + + NAD Reduction Addition of ydrogen 2/Pd or Pt. More difficult compared to alkene hydrogenation, often requiring increased 2 pressure. As a result, any double bonds in the molecule will also be removed. More common industrially. In the lab, hydride donors are used instead. ydride (: - ) Donors ydrides are highly nucleophilic and so you need a partially positive carbonyl carbon for reaction Since a polarized is required they do not reduce double bonds 7
LiAl4 is a hot reducing agent and will actually reduce acids and esters to alcohols. LiAl4 reacts violently with water and decomposes explosively when heated above 120 o NaB4 milder hydride reducing agent, stable in water, reduces aldehydes and ketones quickly (esters very slowly if at all) The principal biologic hydride donor - NAD Question: If NAD + forms NAD when alcohols are oxidized, can NAD reduce aldehydes and ketones back to alcohols? Answer: Yes! If there is no place for the NAD to go namely, a limited amount of 2 available to accept electrons, the reactions carried out by the alcohol dehydrogenases can be reversed. This is what happens during anaerobic exercise and really is the reverse of what we saw above for ethanol oxidation Enz-Nuc- 3 Alcohol dehydrogenase 3 N 2 N 2 N R NAD N + R NAD + The ability of NAD + to reversibly carry : - is one of the truly elegant organic chemistry designs in nature Question: an acetone (2-propanone) be used as a solvent when reducing butanone to 2-butanol with NAD? Answer: 8
Addition of Alcohols But how? I thought we needed to have a good leaving group R + 2 hydrate R + R hemiacetal emiacetal + R atalyst acetal TE biochemistry example of hemiacetal and acetal formation: the cyclization and polymerization of glucose 2 2 Glucose hemiacetal formation 2 2 2 2 2 Glucose hemiacetals condensing to maltose via acetal linkage + + 2 9